(1) Definition of MIMO Technology
A conventional MIMO technology will hereinafter be described in detail.
In brief, MIMO technology is an abbreviation for Multi-Input Multi-Output technology. MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of Tx/Rx data, whereas a conventional art has generally used one transmission (Tx) antenna and one reception (Rx) antenna. In other words, MIMO technology allows a transmission end or reception end of a wireless communication system to use multiple antennas (hereinafter referred to as a multi-antenna), so that the capacity or performance can be improved. For convenience of description, the term “MIMO” can also be considered to be a multi-antenna technology.
In more detail, MIMO technology is not dependent on one antenna path to receive one total message, collects a plurality of data pieces received via several antennas, and completes total data. As a result, MIMO technology can increase a data transfer rate within a specific range, or can increase a system range at a specific data transfer rate. Under this situation, MIMO technology is a next-generation mobile communication technology capable of being widely applied to mobile communication terminals or repeaters. MIMO technology can extend the range of data communication, so that it can overcome the limited amount of transmission (Tx) data of mobile communication systems reaching a critical situation.
(2) System Modeling in MIMO
FIG. 1 is a block diagram illustrating a general MIMO communication system.
Referring to FIG. 1, if the number of transmission (Tx) antennas increases to Nt, and at the same time the number of reception (Rx) antennas increases to NR, a theoretical channel transmission capacity of the MIMO communication system increases in proportion to the number of antennas, differently from the above-mentioned case in which only a transmitter or receiver uses several antennas, so that a transfer rate and a frequency efficiency can be greatly increased. In this case, the transfer rate acquired by the increasing channel transmission capacity can theoretically increase by a predetermined amount that corresponds to multiplication of a maximum transfer rate (Ro) acquired when one antenna is used and a rate of increase (Ri). The rate of increase (Ri) can be represented by the following equation 1.Ri=min(NT,NR)  [Equation 1]
For example, provided that a MIMO system uses four transmission (Tx) antennas and four reception (Rx) antennas, the MIMO system can theoretically acquire a high transfer rate which is four times higher than that of a one antenna system. After the above-mentioned theoretical capacity increase of the MIMO system was demonstrated in the mid-1990s, many developers began to conduct intensive research into a variety of technologies which can substantially increase a data transfer rate using the theoretical capacity increase. Some of the above technologies have been reflected in a variety of wireless communication standards, for example, a third-generation mobile communication or a next-generation wireless LAN, etc.
A variety of MIMO-associated technologies have been intensively researched by many companies or developers, for example, research into an information theory associated with a MIMO communication capacity calculation under various channel environments or multiple access environments, research into a radio frequency (RF) channel measurement and modeling of the MIMO system, and research into a space-time signal processing technology. A mathematical modeling of a communication method for use in the above-mentioned MIMO system will hereinafter be described in detail.
As can be seen from FIG. 1, it is assumed that there are NT transmission (Tx) antennas and NR reception (Rx) antennas. In the case of a transmission (Tx) signal, a maximum number of transmission information pieces is NT under the condition that NT transmission (Tx) antennas are used, so that the transmission (Tx) information can be represented by a specific vector shown in the following equation 2.s=└s1, s2, . . . , sNT┘T  [Equation 2]
In the meantime, individual transmission (Tx) information pieces (s1, s2, . . . , sNT) may have different transmission powers. In this case, if the individual transmission powers are denoted by (P1, P2, . . . , PNT), transmission (Tx) information having an adjusted transmission power can be represented by a specific vector shown in the following equation 3.ŝ=[ŝ1, ŝ2, . . . , ŝNT]T=[P1s1, P2s2, . . . , PNTsNT]T  [Equation 3]
In Equation 3, ŝ is a transmission vector, and can be represented by the following equation 4 using a diagonal matrix P of a transmission (Tx) power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
In the meantime, the information vector Ŝ having an adjusted transmission power is applied to a weight matrix (W), so that NT transmission (Tx) signals (x1, x2, . . . , xNT) to be actually transmitted are configured. In this case, the weight matrix (W) is adapted to properly distribute transmission (Tx) information to individual antennas according to transmission channel situations. The above-mentioned transmission (Tx) signals (x1, x2, . . . , xNT) can be represented by the following equation 5 using the vector (X).
                    x        =                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    ⋯                                                                                      w                                                  1                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    ⋯                                                                                      w                                                  2                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            ⋯                                                                                      w                                                  iN                          T                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            ⋯                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                j                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
Next, if NR reception (Rx) antennas are used, reception (Rx) signals (y1, y2, . . . , yNR) of individual antennas can be represented by a specific vector (y) shown in the following equation 6.y=[y1, y2, . . . , yNR]T  [Equation 6]
In the meantime, if a channel modeling is executed in the MIMO communication system, individual channels can be distinguished from each other according to transmission/reception (Tx/Rx) antenna indexes. A specific channel passing the range from a transmission (Tx) antenna (j) to a reception (Rx) antenna (i) is denoted by hij. In this case, it should be noted that the index order of the channel hij is located before a reception (Rx) antenna index and is located after a transmission (Tx) antenna index.
Several channels are tied up, so that they are displayed in the form of a vector or matrix. An exemplary vector is as follows. FIG. 2 shows channels from NT transmission (Tx) antennas to a reception (Rx) antenna (i).
Referring to FIG. 2, the channels passing the range from the NT transmission (Tx) antennas to the reception (Rx) antenna (i) can be represented by the following equation 7.hiT=└hi1, hi2, . . . , hiNT┘
If all channels passing the range from the NT transmission (Tx) antennas to NR reception (Rx) antennas are denoted by the matrix shown in Equation 7, the following equation 8 is acquired.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    ⋯                                                                      h                                          1                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    ⋯                                                                      h                                          2                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        ⋯                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        ⋯                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Additive white Gaussian noise (AWGN) is added to an actual channel which has passed the channel matrix (H) shown in Equation 8. The AWGN (n1, n2, . . . , nNR) added to each of NR reception (Rx) antennas can be represented by a specific vector shown in the following equation 9.n=[n1, n2, . . . , nNR]T  [Equation 9]
A reception signal calculated by the above-mentioned equations can be represented by the following equation 10.
                    y        =                ⁢                              [                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                    [                                                                                                    h                        11                                                                                                            h                        12                                                                                    ⋯                                                                                      h                                                  1                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                                                  h                        21                                                                                                            h                        22                                                                                    ⋯                                                                                      h                                                  2                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      h                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    h                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            ⋯                                                                                      h                                                  iN                          T                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      h                                                                              N                            R                                                    ⁢                          1                                                                                                                                    h                                                                              N                            R                                                    ⁢                          2                                                                                                            ⋯                                                                                      h                                                                              N                            R                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        j                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                      +                                                                           [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                =                                  Hx                  +                  n                                                                                        [                  Equation          ⁢                                          ⁢          10                ]            
The 3rd Generation Partnership Project (3GPP) supports a type 1 radio frame structure applicable to frequency division duplex (FDD), and a type 2 radio frame structure applicable to time division duplex (TDD).
The structure of a type 1 radio frame is shown in FIG. 3. The type 1 radio frame includes ten subframes, each of which consists of two slots.
The structure of a type 2 radio frame is shown in FIG. 4. The type 2 radio frame includes two half-frames, each of which is made up of five subframes, a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS), in which one subframe consists of two slots. That is, one subframe is composed of two slots irrespective of the radio frame type. DwPTS is used to perform an initial cell search, synchronization, or channel estimation. UpPTS is used to perform channel estimation of a base station and uplink transmission synchronization of a user equipment (UE). The guard interval (GP) is located between an uplink and a downlink so as to remove an interference generated in the uplink due to a multi-path delay of a downlink signal. That is, one subframe is composed of two slots irrespective of the radio frame type.
FIG. 5 is a slot structure of a long term evolution (LTE) downlink. As shown in FIG. 5, a signal transmitted from each slot can be described by a resource grid including NRBDL NSCRB subcarriers and NsymbDL orthogonal frequency division multiplexing (OFDM) symbols. In this case, NRBDL represents the number of resource blocks (RBs) in a downlink, NSCRB represents the number of subcarriers constituting one RB, and NsymbDL represents the number of OFDM symbols in one downlink slot.
FIG. 6 is a slot structure of a long term evolution (LTE) uplink. As shown in FIG. 6, a signal transmitted from each slot can be described by a resource grid including NRBULNSCRB subcarriers and NsymbUL OFDM symbols. In this case, NRBUL represents the number of resource blocks (RBs) in an uplink, NSCRB represents the number of subcarriers constituting one RB, and NsymbUL represents the number of OFDM symbols in one uplink slot.
A resource element (RE) is a resource unit defined by an index (a, b) within the uplink slot and the downlink slot, and represents one subcarrier and one OFDM symbol. In this case, a is an index on a frequency axis, and b is an index on a time axis.
4) Definition of Reference Signal
When a mobile communication system transmits a packet, this transmission packet is transmitted over a radio frequency (RF) channel. As a result, an unexpected distortion may occur in a transmission (Tx) signal. In order to correctly receive the distorted signal described above at a reception end, channel information must be recognized, and the distortion of the transmission (Tx) signal must be corrected by an amount of the channel information. In order to recognize channel information, signals known to both a transmission end and a reception end are transmitted, the degree of distortion of the known signals is detected when the known signals are received over a channel, and finally channel information is recognized on the basis of the detected distortion.
Here, the above signals known to both the transmission end and the reception end are referred to as pilot signals or reference signals.
In recent times, most mobile communication systems use a method for improving Tx/Rx data efficiency using multiple transmission (Tx) antennas and multiple reception (Rx) antennas to transmit a packet, instead of a conventional method of using one transmission (Tx) antenna and one reception (Rx) antenna to transmit a packet. When a transmission end or a reception end of a mobile communication system transmits or receives data using multiple antennas so as to increase capacity or improve a performance or throughput, additional reference signals are present in individual transmission (Tx) antennas, respectively. Signal reception can be correctly carried out under the condition that a channel condition between each Tx antenna and each Rx antenna is recognized.
Provided that M (where M>N) transmission (Tx) antennas can be added to a conventional system including N antennas, a user equipment (hereinafter referred to as a UE) capable of recognizing up to N transmission (Tx) antennas and another UE capable of recognizing up to M transmission (Tx) antennas exist at the same time.
In this case, not only reference signals for supporting N antennas but also M−N additional reference signals must be transmitted. Here, there is a need to effectively transmit data and reference signals under an environment in which a new UE for recognizing M antennas is additionally used without performing additional signaling with an old UE for recognizing only N antennas.